Dynamic range is a measure of the relative brightness of the brightest and darkest parts of an image. Until recently, most televisions, computer monitors and other display devices have been capable of reproducing dynamic ranges of only a few hundred to one. This is far less than the dynamic range that can be appreciated by the human eye. Display devices having greater dynamic ranges are becoming available. Such high dynamic range display devices can provide images that are much more natural and realistic than the images produced by conventional “low dynamic ranges” display devices.
High dynamic range display devices are beneficial in a wide range of applications. For example, high dynamic range display devices may be used for displaying realistic video images ranging from movies and game visuals, to visual displays in simulators such as flight simulators. High dynamic range display devices also have application in demanding imaging applications such as medical imaging.
Many current image data formats specify pixel values using 24 or fewer bits per pixel. These bits specify both the brightness and color for the pixel. 24 bits is too few to specify both a full range of colors and a brightness that can be varied smoothly over the range that a high dynamic range display is capable of reproducing. To obtain full benefit from a high dynamic range display it is necessary to provide image data capable of specifying a wide range of pixel values. Various high dynamic range data formats that provide larger numbers of bits per pixel have been developed or proposed. Such high dynamic range data formats are not typically backward compatible with prior lower dynamic range data formats.
For example, HDRV “Perception-motivated HDR Video Encoding” as described in R. Mantiuk, G. Krawczyk, K. Myszkowski, and H-P. Seidel. Perception-motivated high dynamic range video encoding. ACM Transactions on Graphics (Proc. of SIGGRAPH 2004), 23(3):730-38, 2004 is a lossy HDR video compression method, which, does not offer backward compatibility. The method encodes HDR pixels using 11 bits for luminance and twice 8 bits for chrominance. The resulting video stream does not contain any information on LDR frames.
JPEG HDR is described in Greg Ward and Maryann Simmons. Subband encoding of high dynamic range imagery. In APGV '04: Proceedings of the 1st Symposium on Applied perception in graphics and visualization, pages 83-90, New York, N.Y., USA, 2004. ACM Press. This method involves sub-sampling a subband layer, which can lead to the loss of high frequencies. To prevent it, the method suggest three approaches: pre-correction of LDR layer, to encode within this layer high frequencies that can be lost due to sub-sampling; post-correction which tries to restore high frequencies that has been lost rather than modifying LDR image and full-sampling, which means that no sub-sampling is performed.
There remains a need for practical methods and apparatus for encoding and decoding HDR images, especially HDR video images. There remains a particular need for such methods and apparatus that provide backward compatibility with existing hardware for reproducing lower-dynamic-range images.